In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
The removal of material during metal cutting is by the formation of “chips”. Chatter in metal cutting is generally understood to be the result of regenerative waviness or a varying of actual chip thickness within each chip produced. The self excited form of vibration associated with chatter gains energy from the system due to varying chip thickness. It's unstable and uncontrolled growth can result in significant damage to tools, machines and workpieces. Furthermore, avoiding chatter negatively impacts spindle speeds, depth of cut or both, all of which negatively impact machining efficiencies.
Chatter can be diminished at low RPMs due to “process dampening” that occurs due to the contact of the workpiece behind the cutting edge. Since many metal cutting systems are under damped, this source of energy absorption contributes to stable cutting. A generally accepted schematic of this phenomena is illustrated in FIG. 1, where the change in clearance angle between the insert and the workpiece is depicted. However, under process dampening reduces the efficiency of the metal cutting operation because of contact with the workpiece by non-cutting surfaces of the insert, i.e., rubbing.
The phase relationship in regenerated waviness is depicted in the schematic in FIGS. 2A and 2B for in-phase (FIG. 2A) and out-of-phase (FIG. 2B) conditions. There can be many orders of magnitude in quantity of waves in a single chip generated. Acoustically, this is the noise heard in chatter and is often in the painful hearing range of humans. When the phase relationship is substantially zero (FIG. 2A) stable cutting is experienced and the energy source is essentially zero. When the phase relationship is 180 degrees (FIG. 2B) large cutting force variation and noise are present.
Every combination of spindle on a machine tool, tool holder and material removal tool (a material removal tool system) has unique and inherently stable characteristic frequencies. FIG. 3 illustrates conceptually the concept of stable characteristic frequencies. In FIG. 3, axial depth of cut is plotted as a function of spindle speed. The trace 10 represents the chatter—no chatter transition. Below a certain critical depth of cut (Dcr), all operating spindle speeds result in no chatter. Similarly, as spindle speed becomes low, substantially all depth's of cut result in no chatter. However, at various operating speeds of the spindle, the material removal tool can operate at varying hypothetical depths of cut without inducing chatter, for example, regions 12, 14, 16, 18 and 20, which represent pockets of stability for the material removal tool. The pockets of stability have an integer multiple relationship with respect to the spindle speed. Operating in these pockets of stability would allow operating at a fast stable speed to achieve a high rate of material removal.
Attempts to limit the negative impact on machining efficiencies have been attempted. In one approach, the operating speed of the spindle of the machine tool is empirically selected to be coincident with one of the pockets of stability discussed above and shown in FIG. 3. However, the pockets of stability of a material removal tool may not also be at or even near the maximum operating speed of the spindle, so overall efficiency of cutting is still not maximized. In another approach, preventing chatter at high operating speeds has been attempted by altering the size of the material removal tool. For example, it has been suggested to change the tool overhang length, e.g., the length the tool extends from the holder, to effect the characteristic frequency. However, when operating on a preprogrammed tool path even a slight difference in length from tool to tool can negatively impact machining accuracies and manufacturing efficiencies.